Field of the Invention
The invention relates to a high-strength flat steel product and a method for producing such a flat steel product.
In particular the invention relates to a high-strength flat steel product provided with a metallic protective layer and a method for producing such a product.
Description of Related Art
Where flat steel products are referred to here, this is intended to mean steel strip, sheet or cut sheet metal items obtained from these, such as blanks.
Unless expressly stated to the contrary, in the present text and in the claims the quantities of certain alloying elements are in each case given in wt % and the proportions of certain components of the microstructure in surface percent.
Where in the following cooling or heating speeds or rates are mentioned, then cooling speeds are given in the negative as they lead to a drop in temperature. Accordingly, in the case of rapid cooling, cooling rates have a lower value than for slower cooling. On the other hand, heating speeds leading to an increase in temperature are given in the positive.
Because of their alloying components, high-strength steels have a general tendency to corrode and therefore are typically covered with a metallic protective layer, which protects the respective steel substrate from contact with ambient oxygen. A number of methods for applying such a metallic protective layer are known. These include hot-dip coating, also referred to in the technical jargon as “hot-dip coating”, and electrolytic coating.
Whereas with electrolytic coating the coating metal is deposited electrochemically on the flat steel product to be coated, which in any case becomes slightly heated during the process, in hot-dip coating the products to be coated undergo heat treatment prior to dipping in the respective molten bath. In the process the respective flat steel product is heated under a certain atmosphere to high temperatures, in order to arrive at the desired microstructure and create an optimum surface state for adherence of the metallic coating. Then the flat steel product passes through the molten bath, which similarly is at a raised temperature, in order to keep the coating material in the molten state.
The necessarily high temperatures mean that in hot-dip coating the strength of flat steel products provided with a metallic protective layer has an upper limit of 1000 MPa. Flat steel products with an even higher strength as a rule cannot be hot-dipped, since as a result of the attendant heating resulting from tempering they experience considerable losses in strength. As a result, these days high-strength flat steel products are usually provided with a metallic protective layer electrolytically. This work step calls for a flawless and clean surface, which in practice can only be achieved by pickling prior to the electrolytic coating.
EP 2 267 176 A1 discloses a method for producing a high-strength, cold-rolled strip with a metallic protective coating applied by hot-dip coating, comprising the following work steps:                hot-rolling a hot-rolled strip from a slab,        cold-rolling the hot-rolled strip into a cold-rolled strip,        heat treating the cold-rolled strip, wherein in the course of this heat treatment                    the cold-rolled strip is heated at an average speed of a maximum of 2° C./s from a temperature which is 50° C. lower than the Ac3 temperature of the steel, of which the cold-rolled strip is comprised, to the respective Ac3 temperature,            the cold-rolled strip is then held for at least 10 seconds at a temperature that at least corresponds to the respective Ac3 temperature,            whereupon the cold-rolled strip is cooled at an average speed of a minimum of 20° C./s to a temperature which is 100-200° C. below the martensite start temperature of the respective steel process and            finally the cold-rolled strip is heated for between 1 and 600 seconds to a temperature of 300-600° C.                        
Lastly, the steel strip is hot-dip galvanized. The metallic coating applied here is preferably a zinc coating. Ultimately in this way a cold-rolled strip shall be obtained with optimised mechanical properties, such as a tensile strength of at least 1200 MPa, an elongation of at least 13% and a hole expansion of at least 50%.
The cold-rolled strip processed in the above manner shall comprise a steel, that contains (in wt %) 0.05-0.5% C, 0.01-2.5% Si, 0.5-3.5% Mn, 0.003-0.100% P, up to 0.02% S, and 0.010-0.5% Al, in addition to iron and unavoidable impurities. At the same time the steel shall have a microstructure, having (in surface %) less than 10% ferrite, less than 10% martensite and 60-95% untempered martensite and also 5-20% residual austenite, determined by X-ray diffractometry. Furthermore, the steel can contain (in wt %) 0.005-2.00% Cr, 0.005-2.00% Mo, 0.005-2.00% V, 0.005-2.00% Ni and 0.005-2.00% Cu and 0.01-0.20% Ti, 0.01-0.20% Nb, 0.0002-0.005% B, 0.001-0.005% Ca and 0.001-0.005% rare earth elements.